1. Field of the Invention
The present invention relates to an improved system and method for performing soft handoff between access points of a wireless communications network. More particularly, the present invention relates to a system and method for performing soft handoff using ad-hoc routing and multiple access points of a packet-switched communications network.
2. Description of the Related Art:
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”.
Specifically, a terrestrial cellular network includes a plurality of interconnected base stations that are distributed geographically at designated locations throughout the service area. Each base station includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from user terminals, such as wireless telephones, located in its coverage area. The communications signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, the transceiver and user terminals transmit and receive the data packets in multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base station to communicate simultaneously with several user terminals in its coverage area.
Because each base station can only handle a limited amount of communications signal traffic from the user terminals at any given time, the coverage area of a base station can vary depending on the amount of traffic that the base station is expected to experience. For example, the coverage area of a base station can be set to several miles in diameter in sparsely populated regions, such as rural regions having light wireless traffic, and can be set to less than a mile in diameter in densely populated regions, such as major metropolitan areas having heavy wireless traffic. The wireless communications network therefore must employ many base stations in heavily populated metropolitan areas in order for the network to adequately service the user terminals in those regions.
Each base station is also connected to one or more gateways that enable communication between the base station and other networks, such as the Internet and the public switched telephone network (PSTN). Accordingly, the base stations in the network enable the user terminals to communicate with each other, as well as with other destinations, such as telephony devices, in the PSTN.
Since wireless user terminals are typically mobile, it is common for a user terminal to travel between different base station coverage areas during use. When this occurs, the base station whose coverage area the user terminal is leaving must transfer or “handoff” the user terminal to the base station whose coverage area the user terminal is entering, so that the latter base station can become the base station via which the user terminal and network continue to communicate. In densely populated areas having many base stations with small coverage areas, this handoff process may need to occur several times during a short period of time as the user terminal travels between the different coverage areas.
Many techniques have been developed using the circuit-switched cellular infrastructure to minimize data packet loss during handoff while also minimizing overhead necessary to successfully perform the handoff. For example, a technique known as “hard handoff” refers to a break-before-make technique where the original connection is dropped before the new connection is established. On the other hand, “soft handoff” is a make-before-break technique that maintains multiple simultaneous connections to the user terminal during handoff, and only drops the original connection after the new connection is established. Examples of soft handoff techniques are described in a publication by Wong et al. entitled “Soft Handoffs in CDMA Mobile Systems”, IEEE Personal Communications, December 1997, pp. 6–17, in a publication by Wong et al. entitled “A Pattern Recognition System for Handoff Algorithms”, IEEE Journal on Selected Areas in Communications, Vol. 18, No. 7, July 2000, pp. 1301–1312, and in TIA document TIA/EIA-95-B entitled “Mobile Station-Base Station Compatibility Standard for Wideband Spread Spectrum Cellular Systems”, Feb. 1, 1999, the entire contents of each of these documents being incorporated herein by reference.
With the arrival of the Internet in recent years, some wireless communications networks have moved away from the use of conventional cellular networks and their associated circuit switched routing techniques to improve voice communications services. Each mobile user terminal, telephony device, and any other device capable of communicating with the communications network, has a unique Internet Protocol (IP) address that uniquely identifies it from all other devices. A communications network employing IP sends data between destination points in digital form in discrete packets, rather than in the traditional circuit-committed protocols of the PSTN. Each of the data packets includes the sender's IP address as well as the intended receiver's IP address.
When a wireless user terminal, for example, transmits voice data to a base station of the communications network acting as the access point for the user terminal, a router associated with the base station reads the receiver IP address in the data packet. Each router includes a table of routing information, such as IP addresses of the devices local to the router, available routes, and so on. If the router recognizes from the receiver IP address that the data packet is intended for a telephony device in its immediate neighborhood or domain, the router forwards the data packet to that telephony device. However, if the router does not recognize the IP address as belonging to such a telephony device, the router forwards the data packet to an appropriate adjacent gateway in, for example, the Internet. The router of that gateway then reads the receiver IP address in the data packet, and either delivers the data packet to the appropriate telephony device in its domain, or forwards the data packet to another gateway. Once a router in a gateway recognizes the receiver IP address as belonging to a telephony device in its domain, the router in that gateway delivers the data packet to that telephony device.
It can be also noted that the use of IP to route data packets in a communications network enables the network to handle data other than voice data. For example, such IP techniques can be used to expand the versatility of the network to communicate audio, video or multimedia data between user terminals.
As in traditional cellular communications networks, wireless user terminals in a wireless communications network employing IP can be mobile, and can thus periodically change their access point to the network. Also, wireless user terminals can move outside their “home” network and become temporarily affiliated with a foreign network, and thus communicate via an access point on that foreign network.
Existing mobile data communications networks using IP techniques are based around conventional circuit-switched cellular infrastructures as discussed above, and therefore mask packet routing issues internal to their own networks. That is, when a user terminal moves its affiliation from one access point or base station to another within the network, the network can perform handoff techniques similar to those performed by a conventional wireless cellular network.
However, when a user terminal moves away from its home network and becomes affiliated with an access point on a foreign network, such topology changes must be communicated between routers in the home and foreign networks, so that the routers can update their respective routing tables as necessary, which results in substantial bandwidth-consuming overhead. Accordingly, it is generally agreed that this solution is too slow to handle user terminals that may be constantly mobile between networks, because routing changes must be propagated across the entirety of the connected network.
Another technique to handle the mobility of user terminals between networks without performing router updates is referred to as Mobile IP. A detailed description of Mobile IP is set forth in IETF document RFC 2002 entitled “IP Mobility Support”, October 1996, the entire content of which is incorporated herein by reference. In accordance with the Mobile IP technique, a fixed access point on the mobile user terminal's home network, which can be referred to as a fixed network node, functions as a proxy agent (MIP home agent) for the mobile user terminal (mobile node) when the mobile node moves out of the home network and becomes affiliated with an access point (foreign node) on a foreign network. As understood in the Mobile IP art, the home network is the network at which the mobile node seems reachable to the rest of the Internet or to other networks by virtue of the mobile node's assigned IP address (home address), and a foreign network is the network to which the mobile node is attached when it is not attached to its home network.
Accordingly, when data packets are transmitted from another device to the mobile node, the Mobile IP home agent receives those data packets on the core home network behalf of the mobile node, and encapsulates those data packets. That is, the Mobile IP home agent incorporates each received IP data packet, less any preceding fields such as a media access control (MAC) header, inside another IP data packet, thereby making the fields within the original IP header of the IP data packet temporarily lose their effect. The Mobile IP home agent then tunnels those encapsulated data packet to an agent (Mobile IP foreign agent) on the foreign network to which the mobile node is currently attached. The Mobile IP foreign agent decomposes the capsule and sends the packet to the mobile node, which is currently local to the foreign agent.
Although the Mobile IP technique is reasonably capable of handling the mobility of user terminals between networks, the Mobile IP technique does increase overhead in the networks, since each data packet must be routed to the home network, encapsulated, and then rerouted to the foreign network. Also, the problems solved by Mobile IP can be viewed as those associated with a macro case of mobility in which nodes (user terminals) are mobile between core networks. Mobile IP is unsuitable for handling a micro case of mobility in which user terminals are mobile between wired access points in a single core network.
One solution for managing mobility within a circuit-switched cellular network has been proposed and is referred to as Cellular IP. A description of Cellular IP is set forth in a publication by Andras G. Valko entitled “Cellular IP: A New Approach to Internet Host Mobility”, ACM Computer Communication Review, January 1999, the entire content of which is incorporated herein by reference. Although this solution is somewhat suitable for handling mobility in circuit-switched cellular networks, the technique is specific to the circuit-switched infrastructure of the network.
As can further be appreciated by one skilled in the art, while Mobile IP is suitable for handling cases of user terminal mobility between networks, Mobile IP is deficient in handling problems with packet routing that may occur within a packet-switched core network when a mobile node moves and is handed off from one base station to another. A communications network employing a packet-switched core network is described, for example, in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, the entire content of which is incorporated herein by reference. An example of a wireless local area network (LAN) having mobility is set forth in IEEE Standard 802.11, Aug. 20, 1999, the entire content of which is incorporated herein by reference. Specifically, Mobile IP is incapable of effectively operating under the additional constraints imposed by a packet-switched core network having wired access point nodes that are typically connected using bandwidth-constrained leased lines and which cannot tolerate the additional overhead of twice-routed Mobile IP packets. Furthermore, the low-cost access point nodes also typically cannot tolerate the processor-intensive decomposition of encapsulated packets that is performed during Mobile IP.
In addition to the above deficiencies, other problems involving the manner in which handoff is performed can arise. As explained above, when a user terminal is handed off from one access point to another, the wired network must dynamically reconfigure itself to route packets through the new attachment point. It is often difficult for the wired network to reconfigure itself without the loss of packets sent to the user terminal, because the wired network reconfiguration is not instantaneous. Accordingly, during the reconfiguration period, packets can be sent via both the old and new attachment points. Hence, in existing wireless data networks, one set of these packets will typically be lost.
Furthermore, as the speed of mobility increases, the frequency in which handoff occurs also increases. With this increase in occurrence of handoff, any loss of packets during handoff becomes more noticeable to the user because it occurs more often. Thus, minimizing packet loss during handoff becomes more important as the speed of mobility increases.
Accordingly, a need exists for a system and method capable of effectively and efficiently handling mobility of wireless user terminals between access point nodes of a packet-switched network with minimal overhead and packet loss.